This invention pertains to controlling the temperature of process tools using thermal transfer fluids, and more particularly to meeting the needs of industries which require precise but selectable control of the temperature of units having different thermal loads, such as fabrication equipment using cluster tools for making high precision semiconductors.
Temperature control units for industries which manufacture high precision products, such as multiple semiconductor chips on wafers, must meet a number of stringent and sometimes conflicting requirements. While the manufacture of semiconductors perhaps imposes greater demands than are encountered in most other industrial fields, this industry illustrates particularly well the extent and variety of the problems which might now be encountered with modern temperature control systems. Semiconductor fabrication installations usually include many so-called cluster tools disposed throughout a high cost facility. Wafers are processed using successive steps which demand both high energy usage and close temperature control during removal or addition of thermal energy. Examples of these steps include chemical and high energy deposition and etching procedures carried out in specialized chambers. To maintain the appropriate internal environment and the particular temperature conditions needed for a given process step, separate temperature control units are usually employed to provide a chilled or heated thermal transfer fluid for circulation through the operative process parts of a tool. The temperature control unit must not only maintain the thermal transfer fluid at a prescribed setting and also bring the fluid temperature to its setpoint within specified time limits, but also operate over long periods with very limited down time, be energy efficient and demand minimal floor space.
Preferred systems for such applications have included temperature control units as described in Kenneth W. Cowans U.S. Pat. No. 6,102,113 entitled xe2x80x9cTemperature Control of Individual Tools in a Cluster Tool Systemxe2x80x9d. These temperature control units provide multichannel capability for the control of several different process temperatures by delivery of pressurized refrigerant to chill thermal transfer fluid flows, or by regulated heating of thermal transfer fluids. For refrigerating the thermal transfer fluid, pressurized liquid refrigerant in each channel is passed through an expansion valve regulating flow to an evaporator/heat exchanger. For heating the thermal transfer fluid each channel includes a separate heat source. This temperature control unit employs a single refrigeration unit and single reservoir for the thermal transfer fluid, and uses a different pump in each channel for fluid recirculation. The system has proven to be extremely reliable, requires low floor space (footprint) and provides precise temperature control of the thermal transfer fluid, in both static and dynamic modes.
With time, however, and with the evolution of new cluster tool systems and other units for semiconductor fabrication, a number of additional and particular requirements have more recently been imposed. Thus further and different needs must now be met that necessitate greater flexibility, adaptability and performance, while the goals of long life, compactness and efficient operation remain. For example, some typical modern process tools include more than one unit, such as a process chamber, with each of these having a number of different subunits, each to be brought to and maintained at preset temperature levels. In some of these tools, there may be common settings for like subunits, while other subunits there may be no commonality among the desired settings. Holding temperature at the given levels may require substantial cooling capacity, or only moderate cooling capacity, or even the addition of heat energy. Thermal exchange capacity, usually expressed here in terms of kilowatts, is necessarily a function of both temperature and flow rate.
Overall, the requirements at a semiconductor fabricating facility may differ such that the specified temperature levels can vary from very cold (e.g. down to xe2x88x9240xc2x0 C.), to within a moderate temperature range (e.g. 0xc2x0 C. to 40xc2x0 C.), or to a higher temperature (e.g. up to about 120xc2x0 C. for semiconductor fabrication houses). Moreover, the thermal demand, in KW, may also be substantially different, meaning that the capacity of a compressor or pump, for example, may have to be high for one installation but can be much lower for another. Sometimes one control unit may have sufficient thermal capacity for a number of subunits. In other user environments the temperatures to be maintained may be at more extreme temperature limits, or there may be special needs for varying temperatures within specified time periods.
For most practical applications in the semiconductor fabrication industry, temperature is controlled by circulating a thermal transfer fluid through a cluster tool subunit and back to the temperature control unit, with the user specifying the temperature and flow rate needed. The thermal transfer fluid is typically an equal mixture of ethylene glycol and water, or a proprietary fluid, such as that sold under the trademark xe2x80x9cGaldenxe2x80x9d. These both accommodate very wide differentials between freezing and boiling levels, and have viscosity characteristics which tolerate pumping force differences within the operating temperature limits.
To meet these varied requirements with a compact, low footprint unit is not enough, since it is also desirable to maintain the subunit temperatures while using minimal amounts of energy without losing the flexibility needed to meet temperature level and flow rate requirements for a substantial number of subunits. Cooling solely by air is seldom a viable option. The cheapest available temperature control medium is facilities (utilities) water, for example, which suffices for cooling down to a limited intermediate temperature range somewhat above that of the water itself. For greater chilling capacity, a pressurized refrigerant can be used, while for heating an external thermal energy source, such as an electrical heater, can be employed. Providing appropriate thermal energy solutions for a variety of coexisting needs and at the same time using a compact, high reliability and low energy demand configuration, however, presents problems that have not heretofore been satisfactorily resolved.
In a temperature control unit in accordance with the invention, separate modules of like or related form factors are received in a control chassis, there being at least two broadly distinguishable module types each having at least two different temperature control capabilities, and each with energy savings potential. The modules each have their own pump and reservoir for thermal transfer fluid, an energy efficient unit providing a cooling medium, a heat exchanger or exchangers for transfer of thermal energy between the cooling medium and the transfer fluid, and at least one element for heating the thermal transfer fluid. These modules themselves can be modified while remaining consistent with the defined form factor by the use of differently powered compressors, different capacity pumps, differently sized reservoirs, or more than one heat exchanger. Flow rates as well as thermal load capacities can be adapted or revised to service individual or multiple subunits.
This versatile module-based approach offers a variable array of functionalities to confront the individual needs of multiple operative subunits. Self contained refrigeration loops with thermal transfer fluid reservoirs and pumps enable extraction of heat from a substantial but accommodatable fluid volume in order to cool a process tool. Since the modules can be used in different combinations and internally varied as well, they can be both individually tailored and flexibly responsive to multiple needs on an overall basis. The heat removal rate requirements, which are changeable, of a variety of process tools, can thus be confronted by appropriate module sets, each adapted to meet the temperature level and flow rate needs of individual subunits in the process tools. The basic module types, used in combination, enable control from cooling at low temperatures to heating at relatively high temperatures.
In one type of module, for example, a refrigeration unit is arranged such that compressor energy in a refrigeration loop including an evaporator/heat exchanger can either cool or heat the thermal transfer fluid. This module type cools by expanding pressurized refrigerant in the refrigeration loop or heats using pressurized hot gas from the compressor in a hot gas bypass loop. Heating may additionally be supplied or augmented using the separate heating source in a thermal transfer fluid loop. Thus temperatures can be maintained at different individual prescribed levels with superior energy efficiency in each instance. A second control module type uses a liquid/liquid heat exchanger which receives facilities water as well as thermal transfer fluid, and varies the facilities water flow for mid-range cooling of the process tool unit or subunit. The facilities water flow rate is regulated by a temperature responsive flow control valve combination receiving a control signal from the system. The separate heating source in this unit corrects the thermal transfer fluid temperature rapidly, or independently heats the fluid to a selected higher level.
The entire system is advantageously processor controlled, and includes sensors for detecting the actual thermal transfer fluid temperatures in the different channels that are individually controlled by the modules. A touch screen display enables an operator to enter prescribed operating temperatures and changes, and to review operating values, including fluid flow rates. The physical system configuration is such that a chassis can receive one or more temperature control modules that are integrally sized relative to the standard form factor, such that they removably fit into matingly configured supports or receptacles in the chassis. The modules can be arranged in vertical and/or horizontal arrays, and include front end panels which provide access for adjustments, fluid filling, and draining. Backend panels provide supply and return ports for conduction of thermal transfer fluid through the process tools, and may include couplings for utilities water and electrical power. They also typically include manifolds for coupling thermal transfer fluid lines in common to more than one subunit to be held at the same temperature.
Where a larger compressor or reservoir is to be utilized, this can be accomplished with a module that is a multiple of the standard form factor in width while still being compatible with the control chassis. Where refrigeration capacity needs are less, the refrigeration loop may be simplified as by elimination of features such as a subcooler. If no utilities water is available for cooling, the condenser in the cooling loop may be of an air cooled type. Air conditioning type compressors are typically used, at considerable savings in system cost.
In a specific example of a versatile cooling and heating module, the pressurized refrigerant from the compressor is, for lower temperature chilling applications, liquefied in the condenser and provided through a solenoid controllable expansion valve and a subcooler to an evaporator/heat exchanger, from which expanded refrigerant is returned to the compressor input via the subcooler. The same unit can also be used to heat, moreover, by using a bypass loop from the compressor that is opened when the refrigerant loop conduit is closed at the solenoid expansion valve. Under this condition hot gas refrigerant is directed via a hot gas bypass valve into the evaporator/heat exchanger, heating the thermal transfer fluid to the range of as much as 120xc2x0 C. This bypass loop from the compressor output proceeds through the hot gas bypass valve which opens in response to low pressures at the input to the compressor such as occur automatically when the solenoid expansion valve is shut off. The hot gas bypass loop also safeguards the compressor by returning refrigerant flow to the compressor input when greater input pressure is needed. Advantageously, the hot refrigerant gas is also directed through the reservoir for thermal transfer fluid to increase the temperature of the body of thermal transfer fluid. If desired, the thermal transfer fluid temperature can be increased further or brought more quickly to temperature by activating the electrical heater in the thermal transfer fluid line. This module also may use other expedients, such as employing a desuperheater valve responsive to compressor input temperature to divert a part of the liquid refrigerant from the condenser output to the return input at the subcooler, thus lowering the temperature of the return flow to the compressor.
Additionally, a novel differential pressure valve can be connected into a shunt tubing between the outgoing and return flows of the thermal transfer fluid loop, to prevent over-pressurization by the pump, which particularly can occur with regenerative turbine pumps. A useful indication of the flow rate of thermal transfer fluid is also obtained by a novel flowmeter in one of the lines that is responsive to pressure differentials across an internal orifice. Flow rate readings are often desired by process tool users, if obtainable without undue cost, and reliable over a substantial time period.
For efficient mid-range cooling and alternatively for heating, temperature control can be by controlling facilities water flow using the pressure of gas pressure in an enclosed volume, as determined by a control signal applied to an electrical heater. By signal-regulating the gas pressure in this way, the system opens or closes a pneumatic pressure response flow control valve that controls facilities water flow as needed for regulated cooling of the thermal transfer fluid in a heat exchanger. If the temperature is temporarily lowered too much, it can be brought back up quickly using the electrical heater in the thermal transfer fluid loop. The same heater can be used independently to heat the thermal transfer fluid to a prescribed level. The thermal capacity of this heater (in KW) can be arbitrarily selected by choice of heating elements. The pneumatic controller for the flow control valve includes a gas containing volume thermally coupled to a heater on one side and thermally insulated to a selected degree from the water reference line, to reduce the energy need when heating the bulb and limit the cool down rate when the heater is deenergized.
A novel differential pressure valve in accordance with the invention is virtually noise free and at the same time stable and reliable, and useful to prevent over-pressurization of the thermal transfer fluid loop. It incorporates a spring loaded flexible quill that supports a valve head at one end and merges at the other end into a dashpot slidable within a piston. The valve head is urged by a spring about the quill toward closure against the end face on a conduit for high pressure flow. An adjustment screw, which can be accessible from the exterior of the module, controls the axial position of the piston and therefore the valve opening pressure. The valve opens an exit path from the high pressure conduit to relieve pump pressure by diverting flow to the return line. The flexible quill and dashpot arrangement assures virtually silent operation by damping valve vibrations.
A flowmeter operable with this system comprises a differential capacitive transducer which is coupled to ports on the thermal transfer fluid line that are on the opposite sides of an orifice plate in the fluid flow path. The differential in pressure across the orifice flow path, corrected for flow and viscosity changes by an associated square root circuit, provides an accurate measure of the flow rate that is linear, precise and free from long term drift.